The invention relates generally to panels or sheets of composite material as well as a method of making same and more specifically to a composite material having an inner core disposed between smooth outer panels and intermediate regions of foam as well as a method of making this composite structure.
Many types of plastic composites have been proposed. Both a wide selection of thermoplastic and thermosetting materials have been combined in multiple layers with various intermediate and reinforcing materials to provide many different composite materials exhibiting specific structural characteristics intended for equally specific applications. For example, a composite having a center section of end grain balsa wood disposed between outer resin impregnated fabric layers has been found to provide exceptional strength and rigidity while exhibiting good sound deadening qualities.
A drawback of many such composites, particularly those constituted of only plastic and especially thermoplastic materials, is cold flow or creep. Cold flow or creep is generally defined as that tendency of a structural material which is typically under load to slowly deflect or deform. Although such deformation, to a greater or lesser extent, occurs slowly over extended periods of time, it is generally perceived as permanent inasmuch as only exposure to a reverse load for approximately the same period of time will reverse such deformation. At elevated temperatures, of course, such as experienced by panels or structures exposed to direct, i.e., outdoor, sunlight, creep or cold flow is significantly accelerated.
Such creep or cold flow is a significant disadvantage of many components fabricated of thermoplastic or thermoset materials and frequently limits the use of such materials to applications which are not exposed to outdoor sunlight, relatively high temperatures or significant static loading. This invention is directed to minimizing this drawback of these materials.
The composite structure includes five distinct layers of material: two outer layers or panels, a central rigid core and intermediate foam layers on both sides of the core between the core and the outer panels. The core can be a wide variety of materials but is preferably a rigid foam, aluminum honeycomb or wood having resin or epoxy impregnated fiber or fiber filled outer skins. An array of apertures extends through the core. During the fabrication process, the apertures facilitate cross flow of the liquid foam and ensure even distribution thereof. As a finished product, the foam layers on both sides of the core which extend through the apertures ensure positive location and retention of the core within the foam and ensure conformance to outer surface geometry. The foam may be any suitable injectable foam but is preferably a composition which remains resilient upon curing as such foam more readily accommodates dimensional changes between the layers due to, for example, ambient temperatures changes. The core and foam are surrounded with top and bottom panels of a moldable thermoplastic material such as high density polyethylene (HDPE) or thermoplastic polyolefin (TPO) which is capable of being molded with a high quality exterior finish which may be painted.
A method of fabricating the composite panel material also forms a portion of the present invention. The method includes the steps of providing an upper and lower mold in the shape of the final product and placing a first heated, planar sheet of material such as TPO or HDPE adjacent the lower mold and vacuum forming it. A previously fabricated core is disposed within the interior of the first molded panel and positioned away from it by suitable spacers. The center core, as noted above, includes an array of apertures or passageways and may further include reinforcing members which are positioned to align with mounting members such as threaded fasteners which are used to secure the finished composite structure to hinge members or other mounting components. Then, a second, heated planar sheet of material is positioned adjacent the upper mold and it is vacuum formed. The molds are then aligned and the molds and formed panels are brought together. The mold is then rotated 90 degrees such that the panels and center core are vertically oriented. Expanding foam constitutent(s) are injected into the mold and it is filled to the level of up to approximately ten to twenty percent. The foam then expands and completely fills the interior on both sides of the core and passes through the core passageways as well. Upon curing of the foam, the composite structure is removed from the mold and excess material is trimmed from its periphery.
Products incorporating these features and/or manufactured by this method are highly suitable as cargo and tonneau covers for light trucks and pickup trucks, for vehicle flooring, for truck beds and tailgates and other applications requiring panels having good strength, rigidity and resistance to creep.
It is thus an object of the present invention to provide a composite panel structure having smooth outer surfaces.
It is a further object of the present invention to provide a composite panel structure having a rigid inner core and smooth outer panels.
It is a still further object of the present invention to provide a composite panel structure having outer panels with smooth, grained or painted outer surfaces, a rigid core and intermediate layers of foam between the core and both outer panels.
It is a still further object of the present invention to provide a composite panel structure wherein the core has reinforcing members which accept fasteners for attaching the structure to hinges and other external mounting components.
It is a still further object of the present invention to provide a method for fabricating a composite panel structure.
Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers refer to the same component, element or feature.